Only check for errors in predicate when skipping impl assembly

Prior to PR #91205, checking for errors in the overall obligation
would check checking the `ParamEnv`, due to an incorrect
`super_visit_with` impl. With this bug fixed, we will now
bail out of impl candidate assembly if the `ParamEnv` contains
any error types.

In practice, this appears to be overly conservative - when an error
occurs early in compilation, we end up giving up early for some
predicates that we could have successfully evaluated without overflow.
By only checking for errors in the predicate itself, we avoid causing
additional spurious 'type annotations needed' errors after a 'real'
error has already occurred.

With this PR, the diagnostic changes caused by PR #91205 are reverted.

compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs

@@ -536,7 +536,10 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         // This helps us avoid overflow: see issue #72839
         // Since compilation is already guaranteed to fail, this is just
         // to try to show the 'nicest' possible errors to the user.
-        if obligation.references_error() {
+        // We don't check for errors in the `ParamEnv` - in practice,
+        // it seems to cause us to be overly aggressive in deciding
+        // to give up searching for candidates, leading to spurious errors.
+        if obligation.predicate.references_error() {
             return;
         }